There is known a steering control system for a tracklaying vehicle such as bulldozers, in which a clutch and a brake are provided for each of the right and left drive wheels of the vehicle and controlled to cause right and left turns of the vehicle. In such a steering control system, when operating the steering lever so as to cause a right turn during traveling of the vehicle, the right clutch is disengaged and the right brake is actuated to stop only the right crawler belt, so that the vehicle turns to the right. Similarly, when operating the steering lever to cause a left turn, the left clutch is disengaged and the left brake is actuated to stop only the left crawler belt, so that the vehicle turns to the left.
The above steering control system is designed such that there is a predetermined time interval after clutch disengagement until brake actuation, or after brake release until clutch engagement, which relieves shocks occurring during steering control so that smooth turns can be carried out. If this time interval between clutch disengagement and brake actuation or between brake release and clutch engagement is made to be constant, it will cause a time period when the clutch and brake which have been operated by the steering lever are both in their released states during traveling of the vehicle on a slope, with the result that the vehicle undesirably turns in a direction opposite to a direction designated by operating the steering lever, due to the so-called "reverse steering phenomenon".
An attempt to solve the above problem is disclosed, for instance, in Japanese Patent Publication (KOKAI) No. 4-366232 (1992) in which the time intervals between clutch disengagement and brake actuation and between brake release and clutch engagement are controlled according to the pitch forward or pitch aft angle of the vehicle with intent to prevent shocks occurring during operation of the steering lever and to prevent the reverse steering phenomenon.
A steering control system directed to performing smooth turns without shocks under various driving and operating conditions is disclosed, for instance, in Japanese Patent Publication (KOKAI) No. 5-8753 (1993) according to which, a time interval until a start of brake actuation or until a start of clutch engagement is automatically adjusted according to the operation speed of the steering lever, thereby achieving improved fine controllability.
These conventional steering control systems are, however, not designed to perform steering control taking into consideration the operating state of the tracklaying vehicle such as bulldozer's dozing operation and high speed driving state, and therefore this system cannot provide optimum control suited for each operating state. Another problem of these conventional control systems is such that since the control is designed mainly in view of the static characteristic of the clutches and brakes, it cannot achieve smooth and reliable turning control suited for each operating state.
The present invention is directed to overcoming the foregoing problems and a prime object of the invention is therefore to provide a steering control system for a tracklaying vehicle, which enables optimum steering performance according to various operating conditions of the tracklaying vehicle.